This invention relates to dielectric resonators and, more particularly, to a dielectric resonator apparatus having a plurality of dielectric coaxial resonators unistructurally formed inside a single dielectric block.
In electronic circuits containing resonance circuit sections such as high-frequency band filters (e.g., band pass filters and band rejection filters) or oscillators, it is common to make use of resonance parts using dielectric members (hereinafter referred to as dielectric resonator apparatus) in the resonance circuit section in order to achieve improvements in circuit characteristics and reduction in size. With prior art dielectric resonator apparatus, desired frequency characteristics were obtained by aligning a plurality of dielectric resonators inside a single case and connecting these dielectric resonators by means of external connector elements. Dielectric resonator apparatus thus structured are disadvantageous in that the number of components is large and that they tend to be heavy. For this reason, demands for smaller and lighter dielectric resonator apparatus have been growing. In applications to mobile communication systems such as portable telephones and car telephones, it is particularly important to provide smaller and lighter dielectric resonator apparatus.
The present applicant has earlier proposed a single-body type dielectric resonator apparatus which can be made smaller and lighter than prior art dielectric resonator apparatus (Japanese Patent Application 4-9207). As a background to the present invention, this dielectric resonator apparatus earlier considered but not published, will be described next in some detail with reference to FIGS. 17-22.
FIG. 17 is an external diagonal view of this earlier considered dielectric resonator apparatus when it is about to be completed, having a dielectric block 1 approximately in the shape of a parallelepiped with two throughholes 5a and 5b penetrating therethrough from one end surface 1a to the opposite end surface 1b. An outer conductor 4 is formed nearly all over the outer surfaces of the dielectric block 1, except there are two signal input and output electrodes 9a and 9b also formed on the outer surfaces of the dielectric block 1 but in an electrically insulated condition from the outer conductor 4. The outer conductor 4 is grounded when the dielectric resonator apparatus is mounted on a circuit board (not shown), and the signal input and output electrodes 9a and 9b are connected to signal input and output terminals on this circuit board. As shown in FIG. 18, which is a sectional view of the dielectric block 1 of FIG. 17 across a plane longitudinally cutting across either of the throughholes 5a or 5b, an inner conductor 3 is formed all over the inner surfaces of the throughholes 5a and 5b.
When the dielectric resonator apparatus is in the condition shown in FIGS. 17 and 18, a portion of the inner conductor 3 is removed from the interior of the throughholes 5a and 5b as shown in FIGS. 19 and 20. As shown more clearly in FIG. 19, the portion of the inner conductor 3 which is removed is that portion with a specified longitudinal length t from one end surface 1a of the block 1 inward, thereby leaving an uncovered part .alpha. inside each of the throughholes 5a and 5b. This means that the portion of the outer conductor 4 which is on the (first) end surface 1a is in an electrically "open" (or not contacting) relationship with the inner conductor 3 inside each of the throughholes 5a and 5b, while another portion of the outer conductor 4 on the other (or second) end surface 1b of the dielectric block 1 is electrically connected to the inner conductor 3 inside each of the throughholes 5a and 5b. For this reason, the first and second end surfaces 1a and 1b of the dielectric block 1 may be referred to as the open end surface and the short-circuited end surface, respectively.
FIG. 21 is an exploded view of the dielectric resonator apparatus of FIG. 20 placed upside down and cut horizontally by a plane passing through the central axes of the two throughholes 5a and 5b. In FIG. 21, symbols C.sub.s each indicate the capacitance between a tip part of the inner conductor 3 and the outer conductor 4 formed on the open end surface 1a (hereinafter referred to as the front end capacitance), and symbol C.sub.e indicates the capacitance between each inner conductor 3 and the nearer one thereto of the signal input or output electrode 9a or 9b (hereinafter referred to as the external capacitance). The front end capacitance C.sub.s can be adjusted by varying the length t to thereby control the resonance frequencies of the dielectric resonators as well as the coupling therebetween.
As shown in FIG. 22, which is a circuit diagram of an equivalent circuit of the dielectric resonator apparatus shown in FIGS. 20 and 21, this dielectric resonator apparatus may be regarded as being a structure with dielectric coaxial resonators R1 and R2 respectively formed around the throughholes 5a and 5b and connected in a so-called comb-line manner. The front end capacitance C.sub.s is deemed to be inserted between each of the dielectric coaxial resonators R1 and R2 and the grounded outer conductor 4, and the external capacitance C.sub.e is deemed to be inserted between one of the dielectric coaxial resonators R1 and the nearer one of the signal input or output electrodes 9a and between the other of the dielectric coaxial resonators R2 and the other of the signal input or output electrodes 9b.
Dielectric resonator apparatus which are structured as shown in FIG. 20 with a plurality of dielectric coaxial resonators combined together inside a single dielectric block, are advantageous in that they are generally smaller and lighter than prior art apparatus with individually discrete dielectric coaxial resonators combined together.
The pass band of a dielectric resonator apparatus shown in FIGS. 20 and 21 is determined mainly by the magnitude of the front end capacitance C.sub.s and the pitch S between the two throughholes 5a and 5b. In order to vary the pass band, therefore, one has only to change the magnitude of the front end capacitance C.sub.s and/or the pitch S. It is not possible, however, to change the magnitude of the front end capacitance C.sub.s beyond a certain limit because of physical restrictions. If the pitch S is changed, on the other hand, the external dimensions of the dielectric block 1 are affected, and the size of the dielectric blocks 1 cannot easily be standardized. In other words, dielectric blocks of different sizes will have to be prepared for different frequency characteristics, and this will adversely affect the production cost. If the pitch S is reduced in order to obtain broad band characteristics, the distance between the signal input or output electrode 9a and the throughhole 5a and that between the other signal input or output electrode 9b and the other throughhole 5b become smaller accordingly. As a result, unwanted parasitic capacitance C.sub.x (as shown in FIGS. 21 and 22) becomes too large and the symmetry of the waveforms between the dielectric coaxial resonance elements R1 and R2 becomes poor, adversely affecting the attenuation characteristic of the dielectric resonator apparatus as a whole. If the pitch S is increased in order to obtain narrow-range characteristics, on the other hand, the size of the dielectric block 1 increases, and this defeats the purpose of providing more compact resonator apparatus.